The invention relates to light-emitting devices and light-emitting displays for controlling the light-emitting operations of the light-emitting devices and displaying. More particularly, the invention relates to light-emitting devices such as organic electroluminescence devices having light reflective elements on the back side of an emissive layer and to a technique which is effective when it is used for displays having such light-emitting devices.
An organic electroluminescence device (hereinafter, referred to as an organic EL device) is a device such that by injecting electrons into an emissive layer comprising an organic thin film, an electric energy is converted into a light energy and light is emitted. Unlike a light-emitting display of the non-emissive type represented by a liquid crystal display, since the light-emitting display constructed by the organic EL devices is a self-emissive type, an auxiliary light source such as a back light or the like is unnecessary. Thus, there is a feature such that it is thin and light-weighted, further, a viewing angle is wide, and a response time of display is short.
FIG. 19 is a schematic cross sectional view showing an example of a conventional organic EL device. The organic EL device has a structure such that transparent electrodes 200 serving as an anode, a hole transporting layer 102, an emissive layer 100, an electron transporting layer 101, and a cathode 300 comprising metal electrodes having a light reflecting function are sequentially deposited onto a transparent substrate 400.
When a DC voltage is applied between the transparent electrodes 200 as an anode and the cathode 300, holes injected from the transparent electrodes 200 reach the emissive layer 100 via the hole transporting layer 102, electrons injected from the cathode 300 reach the emissive layer 100 via the electron transporting layer 101, and the electrons and the holes are recombined, so that a light emission having a predetermined wavelength distribution is generated from the recombination region.
In light emitted from the emissive layer 100, the light directing toward the transparent electrodes 200 side passes through the transparent electrodes 200 and is emitted from the transparent substrate 400. The light directing toward the cathode 300 is reflected by the cathode 300, passes through the emissive layer 100, transparent electrodes 200, and the like, and is emitted likewise from the transparent substrate 400.
Therefore, in the display for performing the display by controlling the light-emitting operation of such organic EL devices as mentioned above, a structure such that the cathode is set to an electrode having high reflectance and an amount of light which is emitted from the transparent electrodes side is increased is important in order to obtain a bright image.
According to such a device structure, in a state where no light emission is emitted, since the cathode is in a state like a mirror having high reflectance, an ambient scenery and the like enter, so that a black (dark) display does not sufficiently become dark. That is, there is a problem such that the black display does not become dark under an environment where the ambience is bright, and a contrast ratio decreases. As a method of solving such a problem, a structure such that a circular polarizer 800 is arranged on the light-emitting surface side of the transparent substrate 400 has been put into practical use. The circular polarizer comprises a polarizer 600 and a quarter-wave plate 700.
The circular polarizer 800 operates as follows. An external light 2000 incoming from the ambience into the organic EL devices is generally an unpolarized light. When the light passes through the polarizer 600, a specific linearly polarized light is transmitted and a linearly polarized light which crosses perpendicularly thereto is absorbed. The linearly polarized light transmitted through the polarizer 600 is subjected to an operation of the quarter-wave plate 700 and becomes a circularly polarized light (for example, dextrorotatory circularly polarized light here). When the light which passed through the quarter-wave plate 700 is reflected by the cathode 300, it becomes a circularly polarized light (levorotatory circularly polarized light) whose phase is shifted by π and whose rotational direction is opposite. A light 2000R reflected by the cathode 300 is incoming again into the quarter-wave plate 700. When the light passes therethrough, it is subjected to its operation, converted into the linearly polarized light which is absorbed by the polarizer 600, and absorbed by the polarizer 600, so that it is not returned to the outside. That is, since the external light (incident ambient light) reflected by the cathode 300 is cut, the black display becomes dark and the contrast ratio is remarkably improved.
Such a structure has been disclosed in JP-A-8-509834, JP-A-9-127885, and the like.
Several systems have been proposed and verified with respect to a technique for realizing a full-color image of a display using organic EL devices. For example, the following systems have been proposed: that is, a system in which blue light-emitting devices and a fluorescent CCM (Color-Changing Mediums) are combined (hereinafter, referred to as a CCM method); a system in which a white light emission and color filters of three primary colors of red (R), green (G), and blue (B) are combined (hereinafter, referred to as an RGB-by-white method); a system in which pixels comprising light-emitting devices of three primary colors (R, G, B) are directly patterned (hereinafter, referred to as a direct-patterning approach); and the like.
According to the CCM method, a fluorescent color changing fluorescent dye medius is excited by the light generated in a blue emissive layer and the light is changed from blue to green and red, thereby obtaining 3-primary color light emission. According to this system, if color changing efficiency is low, device efficiency deteriorates. Under a further bright environment, the color-changing mediums is excited by the incident ambient light and becomes bright and the black display is not darkened, so that a contrast ratio deteriorates.
Although the RGB-by-white method has a feature such that the manufacturing is the easiest-because light-emitting devices of only one kind of white are formed, since the color filter is used, using efficiency of the light deteriorates to ⅓ or less in principle.
According to the direct-patterning approach, since it is necessary to form three kinds of devices onto the same substrate, the manufacturing process becomes slightly complicated. However, a loss of light is minimum and the above system is an ideal system from a viewpoint of light emitting efficiency. With respect to the patterning of the RGB, in case of using a material of what is called a small molecules such as fluorescent dye, metalcomplexes, or the like whose molecular weight is small, a technique for finely patterning the RGB by a vacuum evaporation deposition of an organic layer using a shadow mask has been proposed.
In case of using a polymeric material such as π-conjugated polymers, dye-containing polymers, or the like, there has been proposed a technique such that by forming banks of polyimide by photo-etching, a pixel area is separated and an organic material is printed by an ink-jet technology, thereby finely patterning the RGB (the journal of the institute of information and television engineers, Vol. 54, No. 8, pp. 1115 to 1120).
According to the conventional techniques having the circular polarizer, since the incident ambient light reflection by electrodes (cathode) having a light reflecting function of organic EL devices can be reduced by the operation of the circular polarizer, a high contrast ratio can be realized even under the bright environment. However, upon light emission, since a part of the light emitted from the emissive layer is absorbed by the circular polarizer, there is a problem such that the display becomes dark. This is because, since the light emitted from the emissive layer is generally unpolarized light, the light of at least ½ is absorbed by the polarizer constructing the circular polarizer.
In case of realizing the full-color light-emitting displays by the organic EL devices, the direct-patterning approach is most preferable from a viewpoint of the device efficiency. However, in case of the present organic EL devices, a wavelength range of the light emission is broad in dependence on the color and color purity is not high. Although there is a method of further using color filters in order to raise the color purity of each primary color, in this case, since the light is absorbed by the color filters, the using efficiency of the light deteriorates and the display becomes dark.